Owing to the increasing interest in environmental matters there is a great wish to reduce the emissions of pollutants from human activities. The pulp and paper producers are considered culprits in this context. In recent years, however, great resources have been used to reduce the emissions caused by our pulp and paper mills, resulting in great progress.
An important goal that has been strived for is to provide the closed pulp mill, that is to say a pulp mill which minimises emissions by regenerating as much as possible existing chemicals in the process and reusing the resulting spent liquors. One stage is to try to return spent bleach liquors counter-currently to the pulp in the process. A problem arising in connection with this procedure is that certain process-foreign substances, for instance ions of transition metals and alkaline earth metals, which are supplied to the process with, for instance, the wood raw material, may be enriched in the system when spent liquors are being returned.
An increasing quantity of the papermaking pulp is today bleached by means of hydrogen peroxide, peracetic acid or ozone. These bleaching processes are disturbed in different ways by present ions of transition metals. One therefore tries as much as possible to complex these ions of transition metals before bleaching. The chemicals currently used as complexing agents are expensive. There is thus a great need for a method, in which these chemicals can be regenerated in an economical manner.
Large amounts of compounds of transition metals and/or alkaline earth metals can separately, or in combination with each other, cause precipitations on the pulp.
With a view to minimising the enrichment of ions of transition metals and alkaline earth metals, a large number of methods have been presented.
SE 504,424 discloses a method for precipitating transition metals and alkaline earth metals from spent bleach liquors by adding an alkaline liquid. In this method, a green and/or white liquor is added to the spent bleach liquor which is then evaporated with the obtained precipitate remaining in the liquor.
WO 94/232122 discloses a further method for treating process water. An alkaline liquid is added to the process water, whereby the metal ions are precipitated, and then the precipitated metal compounds are separated from the process water.
WO 94/21857 discloses one more method for treating spent liquors from bleach plants. Also in this method, an alkaline liquid is added to precipitate metals. The alkaline liquid is first treated with carbon dioxide to reduce the sulphur content and increase the carbonate content thereof.